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Full text of "Treatise On Applied Analytical Chemistry(Vol-1)"

376

FATTY SUBSTANCES  (GENERAL METHODS)

is kept in a bottle closed with a rubber stopper through which passes a
25 G.C. pipette.

(2)  Seminormal hydrochloric acid.

(3)  Alcoholic phenolpMhalein solution (i% in 95% alcohol).
PROCEDURE.   Inta conical 150-200 c.c. flask, 1-2 grams of the fat are

weighed and treated with 25 c.c. of the alcoholic potash solution, the pipette
being emptied always in the same way. The flask is closed with a stopper
through which passes a glass tube about a metre long to serve as a reflux
condenser and heated on a boiling water-bath, with occasional shaking,
for half an hour or rather more (see Saponification, 5).

The flask is then removed from the bath and the excess of potash remain-
ing free titrated with seminormal hydrochloric acid in presence of 8-10
drops of the phenolphthalein solution.1

A check experiment with 25 c.c. of the caustic potash solution alone
(without fat) is made at the same time and under the same conditions as
the actual test.2

From the difference between the volumes of seminormal hydrochloric
acid used in the check experiment and in the actual test with the fat, the
amount of potash (milligrams) necessary foi the complete saponification
of i gram of the fat is calculated.

EXAMPLE : 1-524 gram of a fat, saponified with 25 c.c. of alcoholic potash,
required 11-9 c.c. of seminormal hydrochloric acid to neutralise the excess of
potash. In the check experiment, 22-5 c.c. of the acid were required for 25
c.c. of alcoholic potash. Since i c.c. N/2-HC1 = 0-02805 gram of KOH, the
amount of KOH necessary to saponify 1-524 gram of fat is (22-5-11-9) 0-02805
gram = 0-2973 gram, so that the amount for i gram of fat is 0-195 gram. The
saponification number of the fat is hence 195.

The saponification number is of importance for distinguishing between
different fats and waxes and especially for the analysis of mixtures of fatty
substances with non-saponifiable matter (mineral oils, resin oils, etc.).

The majority of fatty substances have a saponification number between 190
and 200, but the oils of the Cruciferse (colza oil, ravison oil, etc.), castor oil,
grapeseed oil, and a few other oils, have values below 190.

Coconut oil, palm-kernel oil, certain other vegetable fats, and butter have
numbers above 200. Waxes have very low saponification numbers (below 100).

9. Ester Number

The ester number denotes the number of milligrams of caustic potash
necessary to saponify the neutral fat (neutral esters) in one gram of a fatty
substance. With fats which do not contain free acids, the ester number
is equal to the saponification number ; when, however, free acids are present,

a little powdered calcium carbonate at such a rate that 50 c.c. pass over in 20 minutes.
To test the distilled alcohol, 10 c.c. are boiled with i c.c. of 50 % caustic potash solution
and the liquid allowed to stand for 20 minutes to see if any colour develops. If so,
the alcohol is unsuitable for preparing alcoholic potash and should be again treated
with permanganate.

1  When the fat gives a highly coloured solution with the alcoholic potash, it is
advisable to dilute the liquid considerably with neutral alcohol before titrating, in
order that the neutral point may be determined with accuracy.

2  Two blank experiments and two actual tests should always be made and the
mean of the results taken, provided t hat these do not differ greatly.quid remains clear.r.